1. Technical Field of the Invention
The present invention relates to an optical beam scanning apparatus and an image forming apparatus equipped with the optical beam scanning apparatus, and more particularly, to an optical beams scanning apparatus configured to be capable of adjusting the curve and the inclination of a scanning line for a latent image and an image forming apparatus equipped with the optical beam scanning apparatus.
2. Related Art
An image forming apparatus of the electrophotgraphic method, such as a laser printer, a digital copying machine, and a laser facsimile machine, is equipped with an optical beam scanning apparatus that forms an electrostatic latent image on the photoconductive drum by irradiating a laser beam (optical beam) on the surface of the photoconductive drum and scanning the laser beam thereon.
Recently, a method of increasing the number of laser beams scanned by a single optical deflector (polygon mirror) has been proposed for a tandem color machine. According to this method, plural beams of respective color components (for example, yellow, magenta, cyan, and black) emitted from the corresponding light sources undergo processing in the pre-deflection optical systems, while they are combined into a single beam to go incident on the polygon mirror. The beam deflected on the polygon mirror passes through the fθ lens forming the post-deflection optical system, after which it is separated into beams of the respective color components that are irradiated onto the photoconductive drums of the respective color components.
Incidentally, it is desirable for the optical beam scanning apparatus that the position (loci) of a scanning line in a specific scanning line region on the photoconductive drum always stays at a specific position (loci). However, because of the accuracy of the geometrical arrangement and deformation of the respective optical elements within the optical beam scanning apparatus (for example, a shape error and a posture error of the fθ lens in the post-deflection optical system, and a shape error and a posture error of the post-deflection bending mirror, and so forth), as is shown in FIG. 1A, a scanning line actually curves in the sub-scanning direction as is shown by the scanning line b indicated by a broken line with respect to the ideal scanning line a indicated by a solid line. This phenomenon is referred to as “the curve of the scanning line”.
Herein, “main scanning direction” means a direction in which each laser beam is deflected (scanned) by the optical defector in the optical beam scanning apparatus, and “sub-scanning direction” means a direction orthogonal to both the main scanning direction and the optical axis direction used as the reference of deflection operations provided to a laser beam by the optical deflector for a laser beam deflected (scanned) by the optical deflector to travel in the main scanning direction.
In addition, as is shown in FIG. 1B, the actual scanning line d indicated by a broken line is inclined in the sub-scanning direction with respect to the ideal scanning line c indicated by a solid line. This phenomenon is referred to as “inclination of the scanning line”.
The curve and the inclination of the scanning line as described above have a considerable influence on the image quality of an image to be formed. In particular, they give a significant influence on the quality of an image to be formed in an optical beam scanning apparatus and an image forming apparatus for a 4-consecutive tandem color machine, because the curve and the inclination of the scanning line differ from color to color.
Such being the case, an optical beam scanning apparatus and an image forming apparatus including adjusting means for adjusting the inclination and the curve of the scanning line have been proposed.
According to an optical beam scanning apparatus and an image forming apparatus proposed in JP-A-2004-12596, the center portion of the correction optical element is deformed to be twisted within a plane defined by the main scanning direction and the sub-scanning direction, while one end of the correction optical element is rotated about the fulcrum parallel to the optical axis direction on the other end. This configuration makes it possible to adjust the inclination of the scanning line and the curve of the scanning line.
Also, according to an optical beam scanning apparatus and an image forming apparatus proposed in JP-A-2006-17881, adjusting means for the curve of the scanning line is provided in the center portion of a reflection mirror, which is one of optical members disposed on the optical path of each laser beam. This configuration makes it possible to adjust the curve of the scanning line by a laser beam emitted from the light source with ease.
Further, according to an optical beam scanning apparatus and an image forming apparatus proposed in JP-A-2002-182145, a dell screw is provided to the lens holder to adjust the curve of the scanning line by forcedly bending two long plastic lenses in the sub-scanning direction of a laser beam, and an adjusting screw to correct the inclination of the scanning line by causing the two long plastic lenses to incline integrally with the lens holder is also provided. This configuration makes it possible to adjust both the curves and the inclinations of plural scanning lines.
According to the techniques proposed in JP-A-2004-12596, and JP-A-2002-182145, the optical element of a shape having the curvature in the main scanning direction is deformed. This configuration, however, raises a problem that an influence is provided on the fθ characteristic or the like. In addition, a pressure is applied by the adjusting screw and the dell screw used to twist or bend the long plastic lenses. This configuration, however, poses a problem that the lenses are susceptible to deformation induced by internal stress because they are made of a plastic material.